Structural archaeologist Geoff Carter's radical view of building in the ancient world, especially the archaeology of the lost timber built environment of Southern England. It is new research into of prehistory of architecture, available in a series of articles that are designed to be read in order, and to be accessible to the non-specialist - and there is even some humour

30 April, 2013

Previously, I have discussed the evidence for a temporary timber and earth rampart with associated infrastructure which necessarily predated and facilitated the construction of Hadrian’s Wall in stone, it follows that there was probably a temporary bridge where it crossed the North Tyne at Chollerford, [Chesters].

In addition, unlike a timber bridge built on piles, the
construction of a Stone bridge also requires significant temporary works, which
are evident from the air.

In the context of Hadrian’s Wall there are two aspects to
the crossing of the river;

A carriageway for road traffic

The continuation of the barrier across the river

The surviving bridge abutment on the east bank has two
phases;

An original structure, presumably Hadrian’s, only 3m wide was
designed to carry the wall;

A second and more massive phase, thought to date from the refurbishment
around 160 AD, built to carry both the Wall and the Military Way.[1]

The Fort at Chesters , Bridge Abutment and the North Tyne on Google Earth [2]

Missing Bridges

The evidence from the bridge abutment prompts the question
where was road bridge in the initial for plan Hadrian’s Wall?

I have already suggested that the Vallum was dug as a
foundation for a road behind the Wall, since this flat bottomed trench follows
the ideal course for a road with the spoil moved with enormous effort to
facilitate two lanes on either side of a metalled carriageway. Like much of the
ambitious initial plan, it was abandoned following the Dislocation – a break in
the work presumably caused by warfare. [7]

In addition, it might be presumed there was an initial
timber phase of bridge to carry the Military Way, not to mention some form of
barrier across the river corresponding to the temporary wall. These defencive works would necessarily be to the north of
the Bridge Abutment, while the temporary bridge was perhaps between the between
the Wall and the Vallum.

Another factor would have been any existing bridges in the
area, it is presumed that earlier East – West Stanegate road crossed the North
Tyne a little further downstream.

The area around Chesters on Google Earth

Missing Forts

Chesters Fort is a well preserved stone cavalry fort on the
West bank, which overlies the foundations of an original Wall turret, and in
this form, represents one of the forts that were added to the Wall.

As previously noted, those engaged in the construction and garrisoning
the Wall in its initials phases of construction would require secure
accommodation.

There is a large crop mark which may be such a temporary
fort, [above], with other marks around it, to the south of the Bridge Abutment. There may have been similar arrangements on
the West bank, certainly during the construction of the fort, but perhaps
earlier a military presence on both sides of the bridge may have been
considered prudent.

Missing works

In order to build bridge carried on stone piers in the
river, presumably the water level in the vicinity of these works had to be
lowered to facilitate construction.

There are two possible methods

Divert the river around the works using a channel,

Building individual cofferdams for each pier

While it is clear that Cofferdams were being used by the
early second century, it seems less likely that this technology would be used
by an army in the field on the northern frontier, rather than the technologically
simpler method of diverting the river.[3]
Caesar used a similar method to lower the river Segre, Lerida [Spain] to
allow his troops to ford it, when there was no timber to build a bridge.[4]

The most interesting and feature on the East bank is the ‘L’
shaped feature that forms a two sides of a rough square with the river, around
the bridge abutment, [above]. This appears
to have some interesting characteristics;

Both ends of the feature appear to terminate where the river
bank was in Roman times, slightly further South and East of the existing curve;
[5]

It appears to be blocked, noticeably at the N [upstream] end, as might be expected;

The spoil appears to be thrown to East and South.

The regular island created by this channel is of sufficient
size to accommodate a small fort at the eastern end of the bridge works.

Since the feature’s East West leg follows the presumed line
of Vallum, which was abandoned when work resumed following the dislocation, it
was presumably dug in this or a later period.

It is also appears to be crossed by the line of the wall,
suggesting it dates from early in the construction sequence.

Conclusions

The construction of the Bridge at Chollerford, like the
other more technical aspects of the stonework, would have prioritised for gangs
of specialists, [6] and might be expected that work would be underway by the time
of the Dislocation, when work on the wall was disrupted, having reached this
area.[7]

Since the earliest stone bridge at the existing abutment is
only wide enough to carry the Wall, it adds to the argument that the main road
crossing was intended to be where the ‘Vallum’ crossed the river. In any event,
while the possible channel is perfectly placed to service the Wall crossing and
the later enlarged Wall/road crossing represented by the surviving abutment, it
cuts the presumed line of the Vallum, so could not be related to the
construction of any bridge planned here.

The principle random factor in an understanding of the
engineering of Hadrian’s Wall is the Dislocation, which left work suspended and
then prompted many changes in the scheme; thus, it is possible that this work
had started before, and was completed after the dislocation.

While there should be extensive temporary works which have yet
to be properly defined, the exact position of any early bridge is one of the
more important missing pieces of Hadrian’s Wall at Chollerford.